BigInt: arbitrary-precision integers in JavaScript

BigInts are a new numeric primitive in JavaScript that can represent integers with arbitrary
precision. With BigInts, you can safely store and operate on large integers even beyond the safe
integer limit for Numbers. This article walks through some use cases and explains the new
functionality in Chrome 67 by comparing BigInts to Numbers in JavaScript.

Use cases

Arbitrary-precision integers unlock lots of new use cases for JavaScript.

BigInts make it possible to correctly perform integer arithmetic without overflowing. That by
itself enables countless new possibilities. Mathematical operations on large numbers are commonly
used in financial technology, for example.

BigInt could form the basis of an eventual BigDecimal implementation. This would be useful to
represent sums of money with decimal precision, and to accurately operate on them (a.k.a. the
0.10 + 0.20 !== 0.30 problem).

Previously, JavaScript applications with any of these use cases had to resort to userland libraries
that emulate BigInt-like functionality. When BigInt becomes widely available, such applications
can drop these run-time dependencies in favor of native BigInts. This helps reduce load time,
parse time, and compile time, and on top of all that offers significant run-time performance
improvements.

The native BigInt implementation in Chrome performs better than popular userland
libraries.

“Polyfilling” BigInts requires a run-time library that implements similar functionality, as well
as a transpilation step to turn the new syntax into a call to the library’s API. Babel currently
supports parsing BigInt literals through a plugin, but doesn’t transpile them. As such, we don’t
expect BigInts to be used in production sites that require broad cross-browser compatibility just
yet. It’s still early days, but now that the functionality is starting to ship in browsers, you can
start to experiment with BigInts. Expect wider BigInt support soon.

The status quo: Number

Numbers in JavaScript are represented as double-precision
floats. This means they have limited
precision. The Number.MAX_SAFE_INTEGER constant gives the greatest possible integer that can
safely be incremented. Its value is 2**53-1.

const max = Number.MAX_SAFE_INTEGER;
// → 9_007_199_254_740_991

Note: For readability, I’m grouping the digits in this large number per thousand, using underscores
as separators. The numeric literal separators
proposal enables exactly that for common
JavaScript numeric literals.

Incrementing it once gives the expected result:

max + 1;
// → 9_007_199_254_740_992 ✅

But if we increment it a second time, the result is no longer exactly representable as a JavaScript
Number:

max + 2;
// → 9_007_199_254_740_992 ❌

Note how max + 1 produces the same result as max + 2. Whenever we get this particular value in
JavaScript, there is no way to tell whether it’s accurate or not. Any calculation on integers
outside the safe integer range (i.e. from Number.MIN_SAFE_INTEGER to Number.MAX_SAFE_INTEGER)
potentially loses precision. For this reason, we can only rely on numeric integer values within
the safe range.

The new hotness: BigInt

BigInts are a new numeric primitive in JavaScript that can represent integers with arbitrary
precision. With BigInts, you can
safely store and operate on large integers even beyond the safe integer limit for Numbers.

To create a BigInt, add the n suffix to any integer literal. For example, 123 becomes 123n.
The global BigInt(number) function can be used to convert a Number into a BigInt. In other
words, BigInt(123) === 123n. Let’s use these two techniques to solve the problem we were having
earlier:

BigInt(Number.MAX_SAFE_INTEGER) + 2n;
// → 9_007_199_254_740_993n ✅

Here’s another example, where we’re multiplying two Numbers:

1234567890123456789 * 123;
// → 151851850485185200000 ❌

Looking at the least significant digits, 9 and 3, we know that the result of the multiplication
should end in 7 (because 9 * 3 === 27). However, the result ends in a bunch of zeroes. That
can’t be right! Let’s try again with BigInts instead:

1234567890123456789n * 123n;
// → 151851850485185185047n ✅

This time we get the correct result.

The safe integer limits for Numbers don’t apply to BigInts. Therefore, with BigInt we can
perform correct integer arithmetic without having to worry about losing precision.

A new primitive

BigInts are a new primitive in the JavaScript language. As such, they get their own type that can
be detected using the typeof operator:

typeof 123;
// → 'number'
typeof 123n;
// → 'bigint'

Because BigInts are a separate type, a BigInt is never strictly equal to a Number, e.g.
42n !== 42. To compare a BigInt to a Number, convert one of them into the other’s type before
doing the comparison or use abstract equality (==):

42n === BigInt(42);
// → true
42n == 42;
// → true

When coerced into a boolean (which happens when using if, &&, ||, or Boolean(int), for
example), BigInts follow the same logic as Numbers.

Operators

BigInts support the most common operators. Binary +, -, *, and ** all work as expected.
/ and % work, and round towards zero as needed. Bitwise operations |, &,
<<, >>, and ^ perform bitwise arithmetic assuming a two’s
complement representation for negative values,
just like they do for Numbers.

Unary - can be used to denote a negative BigInt value, e.g. -42n. Unary + is not
supported because it would break asm.js code which expects +x to always produce either a
Number or an exception.

One gotcha is that it’s not allowed to mix operations between BigInts and Numbers. This is a
good thing, because any implicit coercion could lose information. Consider this example:

BigInt(Number.MAX_SAFE_INTEGER) + 2.5;
// → ?? 🤔

What should the result be? There is no good answer here. BigInts can’t represent fractions, and
Numbers can’t represent BigInts beyond the safe integer limit. For that reason, mixing
operations between BigInts and Numbers results in a TypeError exception.

The only exception to this rule are comparison operators such as === (as discussed earlier),
<, and >= – because they return booleans, there is no risk of precision loss.

1 + 1n;
// → TypeError
123 < 124n;
// → true

Note: Because BigInts and Numbers generally don’t mix, please avoid overloading or magically
"upgrading" your existing code to use BigInts instead of Numbers. Decide which of these two
domains to operate in, and then stick to it. For new APIs that operate on potentially large
integers, BigInt is the best choice. Numbers still make sense for integer values that are
known to be in the safe integer range.

Another thing to note is that the >>>
operator,
which performs an unsigned right shift, does not make sense for BigInts since they’re always
signed. For this reason, >>> does not work for BigInts.

API

Several new BigInt-specific APIs are available.

The global BigInt constructor is similar to the Number constructor: it converts its argument
into a BigInt (as mentioned earlier). If the conversion fails, it throws a SyntaxError or
RangeError exception.

Two library functions enable wrapping BigInt values as either signed or unsigned integers,
limited to a specific number of bits. BigInt.asIntN(width, value) wraps a BigInt value to a
width-digit binary signed integer, and BigInt.asUintN(width, value) wraps a BigInt value to
a width-digit binary unsigned integer. If you’re doing 64-bit arithmetic for example, you can use
these APIs to stay within the appropriate range:

Note how overflow occurs as soon as we pass a BigInt value exceeding the 64-bit integer range
(i.e. 63 bits for the absolute numeric value + 1 bit for the sign).

BigInts make it possible to accurately represent 64-bit signed and unsigned integers, which are
commonly used in other programming languages. Two new typed array flavors, BigInt64Array and
BigUint64Array, make it easier to efficiently represent and operate on lists of such values:

The BigUint64Array flavor does the same using the unsigned 64-bit limit instead.

Polyfilling and transpiling BigInts

At the time of writing, BigInts are only supported in Chrome. Other browsers are actively working
on implementing them. But what if you want to use BigInt functionality today without
sacrificing browser compatibility? I’m glad you asked! The answer is… interesting, to say the least.

Unlike most other modern JavaScript features, BigInts cannot reasonably be transpiled down to ES5.

The BigInt proposal changes the behavior of operators (like +, >=, etc.) to
work on BigInts. These changes are impossible to polyfill directly; and they are also making it
infeasible (in most cases) to transpile BigInt code to fallback code using Babel or similar
tools. The reason is that such a transpilation would have to replace every single operator in the
program with a call to some function that performs type checks on its inputs, which would incur an
unacceptable run-time performance penalty. In addition, it would greatly increase the file size of
any transpiled bundle, negatively impacting download, parse, and compile times.

A more feasible and future-proof solution is to write your code using the JSBI
library for now. JSBI is a JavaScript port of the
BigInt implementation in V8 and Chrome — by design, it behaves exactly like the native BigInt
functionality. The difference is that instead of relying on syntax, it exposes an
API: